1. Field of the Invention
The present invention generally relates to a method for forming a semiconductor device, and more particularly to a method for forming BJT (bipolar junction transistor) with high gain via formulation of high voltage device in ULSI (ultra large scale integration).
2. Description of the Prior Art
The bipolar junction transistor (BJT) is an electronic device with two pn junctions in very close proximity. In an one-dimensional bipolar junction transistors (BJT), there are three device regions: an emitter, a base, and a collector. Therein, the two pn junctions are referred to as a emitter-base (EB) and collector-base (CB) junctions. If the emitter and collector are doped n-type and the base is doped p-type, the device is called an npn transistor. If the opposite doping configuration is used, the device is referred to as a pnp transistor.
The BJTs are used as either amplifying or switching devices. In the first application, the transistor's function is to faithfully amplify small ac signals; in the second, a small current is used to switch the transistor from an ON to an OFF state and back.
On the development of deep submicron in ULSI, because the concentration of doping well is high and uniformity, the gain of vertical BJT is decreased. Moreover, the integrated process with forming high voltage device will be limited by the high concentration of well. Thereby, it is hard to integrate the high voltage devices into the deep submicron process.
For the foregoing reasons, there is a need to develop a method for forming bipolar junction transistor (BJT) with high gain to further integrate the high voltage devices into the deep submicron process.